Unmanned aircraft or air vehicles (UAVs) provide enhanced and economical access to areas where manned flight operations are unacceptably costly and/or dangerous. For example, unmanned aircraft outfitted with remotely controlled cameras can perform a wide variety of surveillance missions, including spotting schools of fish for the fisheries industry, monitoring weather conditions, providing border patrols for national governments, and providing military surveillance before, during and/or after military operations.
To perform a surveillance mission, an unmanned aircraft typically carries a remotely operated movable camera. If not stabilized, the pointing direction of the airborne camera swings about as the aircraft maneuvers. Standard practice is to mount such a camera within a gimbal system, and drive the gimbals such that the camera's pointing axis is stabilized. Such an approach can deliver high-quality stabilized pointing, but typically leads to large, heavy, mechanical systems for support of the airborne camera. Such systems are the subject of U.S. Pat. No. 5,897,223, U.S. Pat. No. 3,638,502, U.S. Pat. No. 4,989,466, U.S. Pat. No. 4,643,539 and U.S. Pat. No. 5,184,521.
To fully stabilize an airborne camera typically requires at least three axes of gimbal freedom. Each of the above-mentioned patents describes such a 3-axis system, while U.S. Pat. No. 5,897,223 also describes a 2-axis system. Even with three axes of gimbal freedom, stabilization is degraded in some pointing directions. This occurs when two of the mechanical axes become co-linear, and is known as “gimbal lock.” Stabilization performance can degrade markedly when operating near this “gimbal-locked” configuration. In FIG. 1A of U.S. Pat. No. 5,897,223, for example, gimbal lock can occur when the “horizon” axis is rotated to ±90 degrees from the configuration drawn. FIG. 1B of the same U.S. Pat. No. 5,897,223 depicts a 2-axis gimbal system in which stabilization of the outer axes can be degraded whenever the pointing direction is toward the flight vehicle nadir.
To avoid this degradation in stabilization, existing systems have included additional axes of stabilization internal to the large-motion gimbal axes. One drawback associated with this approach is that the additional inner axes can increase the weight, size, complexity and power consumption of the entire stabilization system.